Antenna system with ferrite radiation suppressors mounted on feed line

ABSTRACT

A high frequency antenna system comprises an antenna, a transmission line such as a coaxial cable electrically connected to the antenna, and a magnetic member surrounding the outer conductor of the coaxial cable. The magnetic member suppresses extraneous currents that are induced in the outer conductor of the coaxial cable by radiations from the antenna. By suppressing the extraneous current in the outer conductor distortion of radiations is prevented. In the preferred embodiment, the magnetic member comprises a plurality of ferrite toroids which collectively form a cylindrical covering on the conductor. Several antenna systems of this type may be closely spaced in a compact array, and each system may operate independently of the others in the array without interference between antennas.

United States Patent Leitner et al. 7

1451 July 25, 1972 [54] ANTENNA SYSTEM WITH F ERRITE RADIATIONSUPPRESSORS MOUNTED ON FEED LINE [72] Inventors: Robert T. Leltner,Norwich; John E.

Drake, Sherburne, both of NY.

[73] Assignee: Jerrold Electronics Corporation, l-latboro,

Pa. 1 [22] Filed: March 2, 1970 211 App]. 110.; 15,612

Primary Examiner-Eli Lieberman Attorney-Sandoe, l-lopgood & CalimafdeABSIRACT A high frequency antenna system comprises an antenna, atransmission line such as a coaxial cable electrically connected to theantenna, and a magnetic member surrounding the outer conductor of thecoaxial cable. The magnetic member suppresses extraneous currents thatare induced in the outer conductor of the coaxial cable by radiationsfrom the antenna. By suppressing the extraneous current in the outerconductor distortion of radiations is prevented. in the preferredembodiment, the magnetic member comprises a plurality of ferrite toroidswhich collectively form a cylindrical covering on the conductor; Severalantenna systems of this type may be closely spaced in a compact array,and each system may operate independently of the others in the arraywithout interference between antennas.

2 Claims, 6 Drawing Figures ANTENNA SYSTEM WITH FERRITE RADIATIONSUPPRESSORS MOUNTED ON FEED LINE This invention relates to highfrequency antennas, and more specifically to means for reducing oreliminating the distortion of radiations usually associated with a longtransmission feed line.

In high frequency transmission systems the transmitting antenna mayoften be located at a considerable distance from the transmitter orcurrent feed source. A long transmission line is therefore required. Acoaxial cable is the most common form of transmission line used for thispurpose as it is structured to feed the current to the antenna with aminimum of signal loss and with no appreciable electromagnetic radiationfrom the line itself. Distortion of radiations, however, is normallyencounted with long transmission lines because stray radiations induceextraneous currents in the line, usually in the outer conductor. Theextraneous and spurious currents are fed to the antenna along with theprimary energy. thereby causing the distortions.

Extraneous currents may be caused by any of several current inducingforces acting on the line. For example, stray radiations from othernearby transmission systems are frequently the cause. Further, theconductivity level of the ground in the vicinity of the coaxial cable isan influencing factor as the outer conductor is grounded.

It is also known that the degree of wave distortion is directly relatedto the effective length of the antenna and the length of transmissionline. For example, the electric field component of a wave radiated froma half-wavelength antenna ideally is a figure 8" pattern, which becomesdistorted to a clover-leaf" pattern as the effective antenna lengthincreases from one-half to three half-wavelengths. Further increases inantenna length are accompanied by further distortions in the electricfield component.

It is most desirable, therefore, to reduce the transmission line lengthas much as possible. oftentimes, however, this is impractical to controland other means of eliminating wave distortion must be used. One commonremedy is that known as a ground plane which is a system of copper wiresburied in the ground in the vicinity of the antenna. This system ofburied wires is effective to alter the conductivity level of the groundand thus stabilize the current flow in the grounded outer conductor ofthe coaxial cable transmission line. While such system is somewhatsuccessful in reducing wave distortion, it is impractical to employbecause of the large physical arrangement of the ground plane and thesubstantial time required to set up the system.

Another technique comprises placing a plurality of high frequency chokesin contact with the outer conductor of the coaxial cable. These chokesare in the form of sections of coaxial cable having a length to presenta high impedance to current flow in the outer conductor. Because aplurality of such chokes must be welded or mechanically attached alongthe cable, the assembly is physically large and cumbersome. Furthermore,because of the nature of this type of choke, it is effective over only asmall operating frequency range.

The problem of wave distortion is particularly important wheremulti-antenna arrays are involved. It is important in array applicationsthat each antenna be isolated from the others.

There has been, therefore, a strong demand for light-weight, compact andefficient means to limit or prevent distortions in the radiationpatterns resulting from the efi'ect of stray radiations on thetransmission line. Isolation is particularly difficult where severalcoaxial cables, feeding the antennas in the array, are in closeproximity to each other and to the antennas. Stray radiations aresubstantial and have a significant effect on the radiation patterns fromthe individual antennas. Further, communication frequencies are now inthe megacycle range requiring physically small antennas, so thatisolation devices must also be small.

Recent efforts to solve this problem have been unavailing. For example,in one assembly, three dipole radiators of a desired size were coaxiallyaligned and spaced from each other by an outer insulating supportingcover. When a high frequency current was transmitted to each dipolethrough a long transmission line, independent operation of the radiatorscould not be maintained because of poor isolation and the resulting highlevel of interference. Lossy material was placed around the transmissionlines between the radiators, but the performance of the radiators wasnot improved. An attempt has been made to arrange the several radiatorsconcentrically instead of in axial relationships but the concentricarrangement was not successful.

Accordingly, it is the primary object of the invention to provide meansfor suppressing extraneous currents in the transmission line of a highfrequency transmission system.

It is another object of the invention to provide a compact andlightweight mechanical assembly in which several independently operatingtransmitting antennas are suongly electrically isolated from each otherover a wide frequency range.

Broadly, the above objects are satisfied and the aforementioneddifficulties are overcome by the use of a magnetic member which at leastpartially surrounds the antenna transmission line. It has beendiscovered that such a member suppresses, or eliminates the extraneouscurrents in the transmission line, and thus radiations from the antennaconnected to the line are substantially undistorted.

In the preferred embodiment of the invention, the magnetic member iscomposed of a ferrite material. The ferrite is formed into a toroid anda plurality of these toroids are placed over the outer conductor of thecoaxial cable to form collectively a cylindrical outer covering on thecable. The toroids are slipped over the end of the cable and moved to asuitable location on the cable until all or a desired part of the cableis surrounded. The use of toroids is desirable as cable flexibility ismaintained.

The novel antenna system has particular utility in a multiantenna arrayin which the several antennas are isolated from each other by themagnetic members and the antenna systems are capable of operatingindependently at different frequencies without mutual interference andover a wide frequency range. In one embodiment, a plurality of antennasare closely spaced and coaxially aligned by a supporting sleeve of aninsulating material. Coaxial cable transmission lines are connected toeach antenna in the conventional manner. The ferrite member surroundseach cable in the space between the adjacent antennas. In this way,isolation is achieved in a compact assembly. In one embodiment, theantennas and insulating sleeve were hollow tubular devices and wereconnected to form a long cylindrical structure. The coaxial cablespassed through the interior of this structure to their respectiveconnection points on the antennas. The structure was relatively light,compact and operable over a wide frequency range, free of mutualinterference.

The combination of a transmission line and a magnetic member surroundingthe line is also useful for other applications in a transmission system.For example, the combination may be employed as a balun. As is known, abalun converts a balanced line into an unbalanced line and vice versa.The operation of a balun involves the transfer of energy e. g. from anunbalanced condition such as the distribution of currents inside acoaxial transmission line, to a balanced condition such as is typical ofthe distribution of current on an open two-wire transmission line.Presently available baluns employ a short circuited quarter-wavelengthopen transmission line at certain terminals to present a high impedanceat those terminals for balancing the transmission lines. A ferritemember surrounding a coaxial cable provides a similar high impedance andtherefore may serve as a balun. With this structure, the frequencysensitive quarter wavelength line is eliminated thereby providing asimple and better stabilized device.

To the accomplishment of the foregoing, and to such other objects as mayhereinafter appear, the present invention relates to an antenna systemas defined in the appended claims and as described in the accompanyingdrawings in which:

FIG. 1 is a schematic illustration of a combination of a coaxialtransmission line and an antenna according to the present invention;

FIG. 2 is a perspective view of the components schematically illustratedin FIG. 1;

FIG. 3 is an approximate graphical illustration of the electrical fieldcomponent of an electromagnetic wave radiated by the antenna of FIG. 1;

FIG. 4 is an approximate graphical illustration of the electrical fieldcomponent of an electromagnetic wave generated by the antenna of FIG. 1without the use of a magnetic member;

FIG. 5 is a cross-sectional view of a multi-antenna array, and

FIG. 6 is a schematicillustration of a balun utilizing the feature ofthe invention.

Referring to FIGS. 1 and 2, there is shown a coaxial transmission line10 surrounded by a magnetic member 12. The member 12 preferably is aferrite sleeve or cylinder or it may be in the form of a plurality offerrite toroids which collectively form a ferrite cylinder. Thetransmission line 10 comprises an inner conductor 16 and an outerconductor 18, which are separated by an insulating material 20. Theferrite cylinder 12 has an inside diameter slightly larger than theouter diameter ofthe coaxial transmission line to ensure a reliably snugfit of the ferrite cylinder 12 the transmission line. An antenna 26 isconnected to the coaxial transmission line 10 to receive high frequencysignals from the line.

Antenna 26 is here shown as a dipole antenna having two equallydimensioned sections 28 and 30 which, as shown in FIG. 2, may bemaintained in a spaced relation by a suitable insulative dielectricspacer 32. The inner conductor 16 of the coaxial transmission line iselectrically connected to section 28, while the outer conductor 18 ofthe transmission line is electrically connected to section 30 by a wire34. Antenna sections 28 and 30 are thereby polarized to conform to therespective potentials on conductors l6 and 18, and radiate highfrequency electromagnetic energy in response to current fed to theantenna by the transmission line. As shown in FIG. 2, coaxialtransmission line 10 is secured to an outer sleeve 36 by a suitableholding means 38 and is provided with a connection 40 at one end whichis adapted to be connected to a current feeding source (not shown).

The radiation pattern of an ideal dipole antenna is illustrated in thebroken-line pattern 42 in FIG. 3, as being generally symmetrical aboutthe zenith and having a minimum radiation along the zenith. Due to thelong length of coaxial line 10, the effective length of antenna 26 isincreased. Without the provision of the magnetic member 12 about theouter conductor of line 10, extraneous currents would be induced in theouter conductor of the coaxial line which causes a distortion of theradiation pattern, and the antenna assembly of FIGS. 1 and 2 would inthat case produce a radiation pattern as illustrated in FIG. 4.

In the antenna assembly of the invention in which the magnetic member 12surrounds the outer conductor of the coaxial line 10, these extraneouscurrents are substantially suppressed or practically eliminated. Theresultant radiation pattern of the antenna assembly of the inventionbecomes the pattern 44 shown in the solid line pattern of FIG. 3. Thevast improvement in the radiation pattern provided by the antennaassembly of the invention due to the provision of the magnetic member isreadily observed by a comparison of the ideal dipole pattern 42 in FIG.3, the realized pattern 44 in FIG. 3, and the radiation pattern obtainedfor the conventional antenna of FIG. 4.

FIG. 5 illustrates a multi-antenna array comprising a plurality ofantennas. The array comprises UHF antennas 45 and 46, and a VHF antenna47 interposed between the two UHF antennas. The three antennas arearranged substantially coaxially and are spaced from one another. Eachof the antennas comprises two axially spaced cylindrical metal sectionswhich are comparable in construction and operation to sections '28 and30 of the antenna assembly of FIG. 1 and 2. Thus antenna 45 comprisessections 50 and 52, antenna 47 comprises sections 54 and 56, and antenna46 comprises sections 58 and 60. Antennas 45, 46 and 47 respectivelyreceive energizing current from coaxial transmission lines 64, 66 and 68which are connected to their associated antennas in the manner describedabove with reference to FIG. 1.

Magnetic members 70 comprising a plurality of ferrite toroids 72 areassembled about each of coaxial lines 64-68 in the spaces between theantennas to achieve satisfactory isolation between the antennas. Theferrite toroids are arranged axially along the coaxial lines andcollectively define a cylindrical magnetic sleeve or cylinder aboutthose portions of the lines located in the spaces between the antennaradiators. The multi-array antenna may be enclosed in a suitablefiberglass support radome 74, and coaxial connectors 76,78 and arerespectively connected to the input ends of coaxial transmission lines64, 66 and 68.

Each of these antenna systems may operate at different frequency rangeswithout adverse affects on the other system. Tests performed on themulti-an'ay antenna system of FIG. 5 indicate that the radiationpatterns of each of the three antennas closely approximates that of asimple dipole in free space as illustrated in FIG. 3. The insertion ofthe ferrite material about the outer shield of the coaxial transmissionlines 64-68 thus effectively eliminates the undesired extraneouscurrents that are induced in the lines. This effect is probably theresult of the increased inductive reactance of the conductor includingthe outer shield of the coaxial line due to the provision of the ferritematerial in the UHF and VHF ranges. Moreover, at frequencies between and440 MHZ, significant isolation between the antenna sections ranging from30db between antennas 45 and 47, to as high as 55db between UHF antennas45 and 46, was obtained.

Another use of the coaxial cable and surrounding magnetic member is as abalun generally designated 84 as illustrated in FIG. 6. The baluncomprises the ferrite cylinder 12a encased about the outer conductor ofa coaxial transmission line 10a. The balun 84 includes balancedterminals a and b and unbalanced terminals 0 and d. These latterterminals would actually be at some distance from ferrite cylinder 12abut are shown adjacent the cylinder for illustrative purposes. Theseveral terminals are connected to the conductors 16 and 18 of thecoaxial cable 14 as shown. In the typical operation of this balun aconversion from the unbalanced terminals 0, d to the balanced terminalsa, b is effected as a result of the high impedance presented atterminals a, b by the ferrite cylinder 12a. The unbalanced condition atremote terminals 0, d is that typically found in the coaxial cablebetween inner conductor 16 and the outer grounded conductor 18. However,the high impedance looking back at terminals a, b is caused by theattachment of the high impedance ferrite cylinder 12a to conductor l8and this inhibits current flow to the normally grounded outer conductor18. As a result, each conductor 16 and 18 is efi'ectively isolated aboveground at terminals 0 and b, thereby causing a balancing of theseterminals relative to ground, which produces a balanced condition on thetwo-wire line.

While the several embodiments shown illustrate the ferrite member as acylindrical tube formed by a plurality of ferrite toroids arrangedaxially about a coaxial transmission line, it will be appreciated thatthe ferrite member may be constructed in any suitable shape providedthat it surrounds at least a part of the outer conductor of the coaxialtransmission line. The ferrite itself may be any one of the commerciallyavailable grades of this material such as TYPE 0-1 or H manufacturer bythe Indiana General Corporation. It is readily manufactured in a toroidor cylindrical shape for use in the systems herein described.

From the foregoing it will be appreciated that by surrounding atransmission line such as a coaxial cable with a magnetic member in anantenna system, a simple solution is provided to a long standingproblem. Excellent radiation pattern generation is assured at highfrequencies both for a single antenna and for a plurality of closelypositioned independently operating antennas. A lightweight, compactmulti-antenna assembly may now be readily provided without wavedistortion caused by the extraneous currents induced in the outerconductors of the coaxial line in the prior art feed systems, andwithout the bulky equipment heretofore required. In addition, themagnetic member and coaxial transmission line may be employed in otherimportant applications in a transmission system such as a balun.

While several embodiments have been described, it will be apparent thatmany other modifications of the invention may be made without departingfrom the spirit and scope of the invention.

I claim:

1. A high frequency antenna assembly comprising at least first andsecond coaxially arranged cylindrical antenna elements for independentlytransmitting electromagnetic waves at respectively different first andsecond frequencies, means for supporting said first and second antennaelements at axially and relatively closely spaced positions relative toone another, first and second coaxial transmission lines arrangedcoaxially with said first and second antenna elements and having acenter conductor coupled respectively to said first and second antennaelements for respectively supplying energizing current to said first andsecond antenna elements, at least one of said first and secondtransmission lines passing through one of said first and second antennaelements to which it is not coupled, and first and second magneticmembers each including a plurality of coaxially arranged ferrite toroidssurrounding the outer shield of said one of said first and secondtransmission lines in the space between said first and second elements.

2. The assembly of claim 1, in which at least a first one of saidantenna means is adapted to transmit electromagnetic waves in the UHFrange and at least a second one of said antenna means is adapted totransmit electromagnetic waves in the VHF range.

1. A high frequency antenna assembly comprising at least first andsecond coaxially arranged cylindrical antenna elements for independentlytransmitting electromagnetic waves at respectively different first andsecond frequencies, means for supporting said first and second antennaelements at axially and relatively closely spaced positions relative toone another, first and second coaxial transmission lines arrangedcoaxially with said first and second antenna elements and having acenter conductor coupled respectively to said first and second antennaelements for respectively supplying energizing current to said first andsecond antenna elements, at least one of said first and secondtransmission lines passing through one of said first and second antennaelements to which it is not coupled, and first and second magneticmembers each incLuding a plurality of coaxially arranged ferrite toroidssurrounding the outer shield of said one of said first and secondtransmission lines in the space between said first and second elements.2. The assembly of claim 1, in which at least a first one of saidantenna means is adapted to transmit electromagnetic waves in the UHFrange and at least a second one of said antenna means is adapted totransmit electromagnetic waves in the VHF range.